John Watson was a man of action. He was a soldier, and a doctor; even if he didn't look like much in his oversized jumpers. So when his flatmate, his best friend, his unrequited love, jumped off the roof of St. Barts he decided to do something about it.
The first thing he did was try to figure out a way, any possible way, that Sherlock might have cheated death. He spent months pouring over that moment in his head, watching footage of the fall some gits had taken with their mobiles and posted online, and covering the flat in photos and post-it notes full of theories. In the end he gave up figuring it out and begged for a miracle over the grave of the most brilliant man he had even known.
After that John grew restless, he could feel the anger of Sherlock's death creeping up on him, he could feel the grief he'd been avoiding begin to seep in. John tried to busy himself, to ignore it, he couldn't help but think that Sherlock would be proud of him for trying to rid himself of the emotions buffeting against him.
"Caring is not an advantage." John understood that now.
"The rest is simply transport." He understood that too.
"All that matters is the work." John needed some. He needed it desperately. Something to busy him, something to give him purpose, and because he was John Watson he needed something to believe in.
The second thing John did was text Greg Lestrade. The Detective Inspector was on a "leave of absence", which basically amounted to house arrest, pending investigation into Sherlock's hoax and resulting suicide. After a few days of round about texting and vague back and forth about stealth tactics and seeing the world, Greg finally agreed to meet John at 221b Baker Street the next day at noon. He hoped to God this wasn't about what he thought it might be. He prayed to a deity he didn't believe in that John Watson wasn't intending on going after Moran and the rest of Moriarty's men, but he packed the extra service pistol the Yard had neglected to confiscate anyway.
John looked as unassuming as ever in a navy jumper with a grayish shirt underneath but his back was ram rod straight and his mouth was a tight line when he smiled at Greg. It occurred to the Inspector that he was no longer looking at Doctor John Watson but at Captain John Watson. The man who was usually so unassuming seemed to demand the attention of the entire room as he stood, and even though Greg was the only other person present at 221b Baker Street that afternoon he was certain that all eyes would have been resting warily on John had that not been the case. Watson was several inches shorter than Lestrade but Greg felt as though the other man were towering over him, and he had to swallow twice before speaking.
"So what's this really about then John?"
"You know exactly what this is all about Greg." John replied coolly and without inflection as he turned to fix the clasp on his bag.
"Revenge?"
"Justice." John said turning sharply to face him. "It's about justice."
"For Sherlock?"
"Of course for bloody Sherlock! Who do you think?" John raised his voice suddenly startling Greg, but more alarming was the calm monotone which he dropped back into with startling ease. "He wasn't a fake Greg. He wasn't perfect but he wasn't a fake, and he didn't deserve to die like that."
John turned to Greg and the lost look that flashed across his face tore at the wounds Greg had been trying to heal since dispatch had contacted him about the incident.
"John we can't just turn into vigilantes." He said blankly.
"You're a protector of the people. These men are going to hurt more people. They're not going to stop without Jim, if anything they're more of a threat without someone to pull the strings and keep them in line. It's not just about vengeance Greg; it's the right thing to do. It's something I need to do, and I can't do it alone."
"I'm going to regret this." Greg said as he might have to Sherlock once. John smiled and Greg began to regret his decision almost instantly.
"I won't."
They took the train out of the city and into Paris using their real names and IDs. Then looping back around almost immediately they switched to the fake IDs John had procured for them and headed out of France. A change of clothes and a poor gypo who promised to check into a hotel using John's credit card ensured that Mycroft wouldn't catch their trail for a least a few hours. A few hours were all it took to disappear.
The game was on.